Ferritic stainless steel plate with Ti and method for production thereof

ABSTRACT

A Ti-containing ferritic stainless steel sheet and a manufacturing method thereof include stainless steels being formed while a refining load is decreased and having a low yield strength which exhibits superior workability. The Ti-containing ferritic stainless steel sheet contains on mass percent basis: 0.01% or less of C; 0.5% or less of Si; 0.3% or less of Mn; 0.01% to 0.04% of P; 0.01% or less of S; 8% to 30% of Cr; 1.0% or less of Al; 0.05% to 0.5% of Ti; 0.04% or less of N, 8≦Ti/(C+N)≦30 being satisfied; and the balance being substantially Fe and incidental impurities, wherein a grain size number of ferrite grain is 6.0 or more, and an average diameter Dp of precipitation diameters, each being [(a long axis length of a Ti base precipitate+a short axis length thereof)/2], of the Ti base precipitates in the steel sheet is in the range of from 0.05 μm to 1.0 μm.

TECHNICAL FIELD

The present invention relates to Ti-containing ferritic stainless steelsheets having a low yield strength which exhibits superior workabilityand to manufacturing methods thereof. In particular, the presentinvention relates to hot rolled and cold rolled Ti-containing ferriticstainless steel sheets and manufacturing methods thereof, each ferriticstainless steel sheet having a structure made of fine grains and a lowyield strength which exhibits superior workability preferably used forapplications in which a high r value and high ductility are required.

BACKGROUND ART

As methods for improving the workability of ferritic stainless steelsheets, for example, a method has been disclosed in Japanese UnexaminedPatent Application Publication No. 3-264652 in which Ti or Nb is addedbesides reduction of C and N. In addition, in Japanese Unexamined PatentApplication Publication No. 5-320772, as methods for manufacturing amore inexpensive Ti-containing ferritic stainless steel sheet, amanufacturing method has been disclosed in which besides hot rollingcontrol performed by high temperature coiling, precipitation of FeTiP,which causes hardening and decrease in ductility, is suppressed bycontrolling the contents of P, S, C and N so that annealing ofhot-rolled steel sheets can be omitted.

As is the above method, in Japanese Unexamined Patent ApplicationPublication No. 10-204588, a method for manufacturing stainless steelsheets having superior workability has been disclosed. In this method,the upper limits of P, C, S, and N contents forming phosphides,carbides, nitrides, and sulfides with Ti are controlled for suppressingthe precipitation of phosphides, carbides, and sulfides in coiling of ahot-rolled steel sheet so as to facilitate recrystallization in thecoiling mentioned above, and as a result, although annealing of ahot-rolled steel sheet is omitted, a stainless steel sheet havingsuperior workability can be manufactured. In the above threeconventional techniques, P and C precipitates, and P and C in a solidsolution form are regarded as elements harmful to the workability, andit has been believed that it is important to reduce the contents of Pand C as small as possible by refining.

The reduction of P and C contained in steel by refining described aboveis effective for improvement in steel properties; however, the reductiondescribed above may cause some problems in some cases. For example, thefollowing may be mentioned.

-   (1) When recycling of dust and slag by-produced in a    steel-manufacturing process and reuse of scrap are taken into    consideration, in order to reduce the contents of P and C in steel,    which inevitably come from the starting materials mentioned above,    to a predetermined level, refining for a long period of time is    required in a steel-manufacturing process, and as a result, the    productivity is decreased.-   (2) By the reduction of the elements described above, it becomes    difficult to control the growth of steel grains, and concomitant    with larger and coarser grain diameters of a hot-rolled steel sheet,    the anisotropy is increased, resulting in apparent generation of    ridging (surface irregularities) and the like.

An object of the present invention is to provide stainless steel and amanufacturing method thereof, the stainless steel having improvedworkability and properties such as a yield strength. In the stainlesssteel described above, P present therein is allowed to remain to acertain extent by refining so that a load required for refining isdecreased, and in addition, P in the form of larger and coarser Ti baseprecipitates is positively precipitated so as to make P harmless. Inaddition, an object of the present invention is to enable existingmachines to be efficiently used without enhancing the capacities thereofand is to achieve recycling of steel materials and energy saving inmanufacturing.

DISCLOSURE OF INVENTION

The aspects of the present invention are as follows.

That is, the present invention provides a Ti-containing ferriticstainless steel sheet comprising on mass percent basis: 0.01% or less ofC; 0.5% or less of Si; 0.3% or less of Mn; 0.01% to 0.04% of P; 0.01% orless of S; 8% to 30% of Cr; 1.0% or less of Al; 0.05% to 0.5% of Ti;0.04% or less of N; and the balance being substantially Fe andincidental impurities, in which 8≦Ti/(C+N)≦30 is satisfied. In theTi-containing ferritic stainless steel sheet, the grain size number offerrite grain is 6.0 or more, and an average diameter Dp ofprecipitation diameters, each being [(a long axis length of a Ti baseprecipitate+a short axis length thereof)/2], of the precipitates in thesteel sheet is in the range of from 0.05 μm to 1.0 μm. In addition, inthe Ti-containing ferritic stainless steel sheet described above, atleast 50% of the total Ti content in the steel sheet is precipitated inthe form of the Ti base precipitates (phosphides, carbides). Inaddition, in the Ti-containing ferritic stainless steel sheet describedabove, at least 50% of the total P content in the steel sheet isprecipitated in the form of the Ti base precipitates. In addition, theferritic stainless steel sheet described above includes a hot-rolledsteel sheet and a cold-rolled steel sheet.

In addition, the present invention provides a method for manufacturing aTi-containing ferritic stainless steel sheet, which comprises the stepsof: hot-rolling steel which contains on mass percent basis: 0.01% orless of C; 0.5% or less of Si; 0.3% or less of Mn; 0.01% to 0.04% of P;0.01% or less of S; 8% to 30% of Cr; 1.0% or less of Al; 0.05% to 0.5%of Ti; 0.04% or less of N; and the balance being substantially Fe andincidental impurities, in which 8≦Ti/(C+N)≦30 is satisfied, for forminga hot-rolled steel sheet, and performing recrystallization annealing ofthe hot-rolled steel sheet at a temperature in the range of (aprecipitation nose temperature T of Ti base precipitates±50° C.) so thatan average diameter Dp of precipitation diameters, each being [(a longaxis length of a Ti base precipitate+a short axis length thereof)/2], ofthe Ti base precipitates in the steel sheet is in the range of from 0.05μm to 1.0 m and so that a grain size number of ferrite grain is 6.0 ormore. Alternatively, in addition, the method for manufacturing aTi-containing ferritic stainless steel sheet, described above, mayfurther comprise the steps of: cold-rolling the hot-rolled annealedsteel sheet thus obtained; and subsequently performing final(recrystallization) annealing of the cold-rolled steel sheet at atemperature less than (a precipitation nose temperature T of Ti baseprecipitates+100° C.) and preferably at a temperature less than (theprecipitation nose temperature T of Ti base precipitates+50) so that theaverage diameter Dp of precipitation diameters, each being [(a long axislength of a Ti base precipitate+a short axis length thereof)/2], of theTi base precipitates is in the range of from 0.05 μm to 1.0 μm and sothat the grain size number of ferrite grain is 6.0 or more andpreferably 6.5 or more. In addition, in the method for manufacturing aTi-containing ferritic stainless steel sheet, at least 50% of the totalTi content in each of the hot-rolled steel sheet and the cold-rolledsteel sheet is precipitated in the form of the Ti base precipitates. Inaddition, in the method for manufacturing a Ti-containing ferriticstainless steel sheet, at least 50% of the total P content in each ofthe hot-rolled steel sheet and the cold-rolled steel sheet isprecipitated in the form of the Ti base precipitate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship of an average diameter Dp(μm) of Ti base precipitates with an average r value and the elongation(%)

FIG. 2 is a graph showing the relationship of a grain size number (Gs.No.) of a cold-rolled annealed steel sheet with Δr (anisotropy) andsurface roughness (μm) thereof.

FIG. 3 is a graph showing the relationship between a grain size number(Gs. No.) of a hot-rolled annealed steel sheet and the yield strength(MPa) of a cold-rolled annealed steel sheet.

FIG. 4 is a TTP curve (schematic view) of Ti base precipitates(carbides, phosphides) of a hot-rolled annealed steel sheet.

FIG. 5A shows the appearance (TEM/replica) of Ti base precipitatesobtained by a conventional annealing condition for a hot-rolled steelsheet.

FIG. 5B shows the appearance (TEM/replica) of Ti base precipitatesobtained by an annealing condition for a hot-rolled steel sheet of thepresent invention.

FIG. 6A shows the appearance (TEM/replica) of Ti base precipitatesobtained by a conventional intermediate annealing condition (continuousannealing).

FIG. 6B shows the appearance (TEM/replica) of Ti base precipitatesobtained by an intermediate annealing condition of the presentinvention.

FIG. 7A shows the appearance (TEM/replica) of Ti base precipitatesobtained by a conventional final annealing condition (continuousannealing).

FIG. 7B shows the appearance (TEM/replica) of Ti base precipitatesobtained by a final annealing condition of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In order to achieve the objects described above, the inventor of thepresent invention carried out detailed investigation of influences ofprecipitation behaviors of carbides and phosphides on the qualities of acold-rolled annealed steel sheet by variously changing the P content ofcommercially available process materials. According to the results,instead of reducing the P content in steel as small as possible tosuppress the precipitation of carbides and phosphides, in considerationof recycling of slag and dust, when the P content is allowed to remainin an appropriate amount as a starting material in a steel refining stepso that the load required for refining is decrease, and when the sizeand amount of Ti base precipitates in a steel sheet and the grain sizenumber of ferrite grain thereof are controlled in predetermined ranges,it was found that without reducing the P content as small as possible,the ductility and the r value of a hot-rolled and a cold-rolled sheetare improved.

Specifically, in order to achieve the objects described above, bymeasuring the amounts of Ti precipitates at various annealingtemperatures (500° C. to 1,000° C. at regular intervals of 25° C.) andfor various annealing times (1 minute, 10 minutes, 1 hour, and 100hours) using hot-rolled ferritic stainless steel sheets having various Pcontents (0.04% of C, 0.10% of Si, 0.25% of Mn, 0.013% to 0.046% of P,0.003% of S, 16.2% of Cr, 0.02% of Al, 0.16% of Ti, and 0.008% of N),the inventor of the present invention obtained the range in which theamount of the Ti precipitates was at least 50% of the Ti content in thesteel sheet, and subsequently, a TTP curve (curve showing therelationship among temperature, time, and precipitation/precipitationstart curve) as shown in FIG. 4 was prepared. A temperature at a noseportion in FIG. 4 was represented by N and was defined as aprecipitation nose temperature T (° C.) of Ti base precipitates(carbides, phosphides, and the like). In addition, after the hot-rolledsteel sheets were annealed at various temperatures (500° C. to 1,000° C.at regular intervals of 25° C.) and for various annealing times (1minute, 10 minutes, 1 hour, and 100 hours), from the change in hardnessand the observation of the structures, recrystallization behaviors wereinvestigated. From these measurement results, that is, by overlappingthe TTP curve of Ti base precipitates and the relationship of therecrystallization behaviors, appropriate annealing conditions could befound for individual types of steel in which the precipitates wereeasily obtained and in which the recrystallization could be completed.In the TTP curve described above, the temperature and the logarithm oftime were plotted in ordinate and in abscissa, respectively, and thecontour line was drawn in which at least 50% of the total Ti content inthe steel sheet was precipitated, thereby forming the precipitationcurve.

In addition, the ratio of a part of the total Ti content in each of thehot-rolled annealed steel sheet and cold-rolled annealed steel sheet,which was precipitated in the form of the Ti base precipitates, wasobtained by multiplying 100 and an analyzed amount (mass percent) of theTi precipitates in steel divided by the total Ti content (mass percent)therein. “The total Ti amount (mass percent)” was measured in accordancewith JIS G1258: 1999 (Iron and steel-Methods for inductively coupledplasma atomic emission spectrometry). That is, a sample is dissolved inan acid (hydrochloric acid+nitric acid). After a residue is recovered byfiltration and is processed by an alkaline fusion (sodiumcarbonate+sodium borate), the residue thus processed is dissolved inhydrochloric acid and is mixed together with the acid solution mentionedabove, and the mixture thus obtained is diluted with purified water to apredetermined volume. Subsequently, by an ICP emission spectrometer, theTi amount (TiA) in this solution is quantified.Total Ti amount (mass percent)=TiA/sample weight×100

“The precipitated Ti amount (mass percent)” is obtained byconstant-current electrolysis (current density of 20 mA/cm² or less) ofa sample using an acetyl acetone base electrolyte (a so-called AAsolution). A residue in the electrolyte after this electrolysis isrecovered by filtration and is processed by an alkaline fusion (sodiumperoxide+lithium methaborate), and then the residue thus processed isdissolved by acid and is diluted with purified water to a predeterminedvolume. Subsequently, by an ICP emission spectrometer, the Ti amount(TiB) in this solution is quantified.Precipitated Ti amount (mass percent)=TiB/sample weight×100

In addition, the form (size, distribution, and amount) of the Ti baseprecipitates of the hot-rolled annealed steel sheet were investigated byvarious changing precipitating temperatures T and times ofrecrystallization annealing. Furthermore, after this hot-rolled annealedsteel sheet was cold-rolled, recrystallization annealing (finalannealing) was performed at various temperatures, and the relationshipamong the size of the Ti base precipitates in the final cold-rolledsteel sheet, the yield strength (hereinafter referred to as “YS”), andthe grain size number was investigated.

According to the results, instead of reducing P in steel as small aspossible by refining so as to suppress the precipitation of the Ti baseprecipitates, it was found that when the P content is allowed to remainin an appropriate amount in steel, and when at least 50% of Ti thereinis subsequently precipitated in the form of large and coarse Tiprecipitates as Ti precipitates having an appropriate size in a step inwhich the hot-rolled steel sheet is annealed, P and C dissolved in steelcan be decreased so as to make P and C harmless and that at the sametime a matrix can be purified. In addition, compared to a conventionallow YS material having larger and coarser crystal grains due tohigh-temperature final annealing, it was found that a low YS materialhaving a remarkably fine structure can be obtained.

That is, the observation results of Ti base precipitates of a hot-rolledannealed steel sheet, an intermediate-annealed steel sheet, and afinal-annealed steel sheet obtained under conventional annealingconditions and the annealing conditions of the present invention areshown in FIGS. 5A, 5B, 6A, 6B, 7A, and 7B. In the annealed materialsunder the conventional annealing conditions, the size of the Ti baseprecipitates finely precipitated in the hot-rolled annealed steel sheetis gradually increased in subsequent annealing of a cold-rolled steelsheet (intermediate annealing and final annealing) (see FIGS. 6A and7A); however, on the other hand, unlike the case described above, in theannealed materials containing the Ti base precipitates according to thepresent invention, large and coarse precipitates are gradually dissolved(see FIGS. 6B and 7B). In addition, in the hot-rolled annealed steelsheet obtained under the conventional annealing conditions, elementssuch as P and C in a solid solution form remain in the matrix, andfurthermore, since the Ti base precipitates are fine, the tensilestrength (hereinafter referred to as “TS”) is high, and the ductility ispoor. Fine precipitation of the Ti base precipitates insufficientlyperformed by subsequent heat treatment hardens steel.

According to the present invention, (1) Ti base precipitates (carbides,phosphides) in a hot-rolled steel sheet are precipitated in a large andcoarse form at a low density by precipitate annealing; (2) elements suchas P and C in a solid solution form are decreased thereby, andconcomitant with the improvement in purity of a matrix and with theformation of the larger and coarser Ti base precipitates at a lowerdensity, a recrystallization temperature of a cold-rolledintermediate-annealed steel sheet is decreased; and (3) by annealing ofthe cold-rolled sheet at a low temperature, redissolution of the Ti baseprecipitates (phosphides, carbides) in the hot-rolled steel sheet issuppressed (a recrystallization temperature of a final-annealed sheet isalso decreased by the same mechanism as described above). Accordingly,as the C and P in a solid solution form are decreased, since theprecipitates grow large and coarse and have a low density as compared tothe conventional annealed material, (4) in the cold-rolled annealedsteel sheet, a low YS, a low TS, a high elongation (hereinafter referredto as “ductility El”), and a high r value can be achieved.

Hereinafter, individual important points of the present invention willbe described. First, the contents of individual elements of theTi-containing ferritic stainless steel sheet will be described. In thepresent invention, the content of each component is represented by masspercent and may be simply represented by % in some cases.

(1) C: 0.01% or Less:

When C is contained in a solid solution form, steel is hardened (solidsolution reinforcement). In addition, C precipitates in the form of Crbase carbides and is primarily located in grain boundaries, resulting indegrading secondary cold-work embrittlement and corrosion resistance ofthe grain boundaries. In particular, when the content is more than0.01%, the influence becomes significant, and hence the content islimited to 0.01% or less. In addition, in consideration of the loadrequired for refining and control of precipitates, the content ispreferably in the range of from more than 0.002% to 0.008%.

(2) Si: 0.5% or Less:

Si is an effective element for improving oxidation resistance andcorrosion resistance and improves the corrosion resistance in theatmospheric environment. In addition, Si is used as a deoxidizing agentfor removing oxygen in steel. However, when the Si content is increased,concomitant with the increase of Si in a solid solution form, steel ishardened (solid solution reinforcement), and the ductility is alsodecreased. Accordingly, the upper limit of the content is set to 0.5%.The content is preferably in the range of from 0.05% to 0.2%.

(3) Mn: 0.3% or Less:

Mn is an effective element for improving oxidation resistance; however,when it is excessively contained, the toughness of steel is degraded,and resistance against secondary cold-work embrittlement of a weldedportion is also degraded. Accordingly, the content is set to 0.3% orless. The content is preferably in the range of from 0.15% to 0.25%.

(4) P: 0.01% to 0.04%:

P is concentrated in grain boundaries and makes steel brittle. Inaddition, when being dissolved in a solid solution form, P remarkablyhardens steel and degrades the ductility thereof. Furthermore, the Pcontent is preferably low in view of resistance against cold-workembrittlement of a welded portion and of high-temperature fatigueproperties. However, when recycling of various starting materials usedin a steel-manufacturing process is considered, excessive reduction in Pcontent may result in increase in steel-manufacturing cost. In addition,when the P content is decreased, the size of the Ti base precipitates isdecreased. In addition, by strain caused by hot rolling, the stabilityof the precipitates is decreased. In addition, when the volumes of theprecipitates are equivalent to each other, a small precipitate having ahigh density has a higher capability of hardening steel than a largeprecipitate having a low density; hence, the control of precipitates isimportant so as to have a large and coarse form having a low density.Accordingly, in order to form P into relatively large and coarseprecipitates present in a hot-rolled annealed steel sheet, it isimportant to allow an appropriate amount of P to remain.

In addition, when the P content is more than 0.04%, since corrosionresistance and toughness are seriously degraded, the upper limit is setto 0.04%. In addition, in view of the load required for refining steel,recycling of refined dust, slag, and scrap in a steel-manufacturingprocess, and the control of precipitates, a content of from 0.01% to0.04% is set as an appropriate range. In consideration of the loadrequired for refining and recycling described above, the P content ispreferably in the range of from 0.020% to 0.030%.

(5) S: 0.01% or Less:

S degrades the corrosion resistance of steel. However, since S canstabilize C in a solid solution form in steel at a high temperature as astable precipitate in the form of Ti₄C₂S₂, even when S is contained to acertain extent, serious problems may not occur. Accordingly, inconsideration of an economical load for desulfurization in steelmanufacturing, the content is set to 0.01% or less. The content ispreferably in the range of from 0.002% to 0.006%.

(6) Cr: 8% to 30%:

Cr is an effective element for improving corrosion resistance. However,in order to ensure sufficient corrosion resistance, the content must be8% or more. In addition, in order to ensure higher-level corrosionresistance such as that required in a seaside environment or at a weldedportion, a content of 11% or more is preferable at which a passivationfilm becomes stable. On the other hand, Cr is an element degrading theworkability of steel, and in particular, at a content of more than 30%,the influence becomes apparent. Furthermore, due to an effect combinedwith another element, steel becomes brittle by precipitation of a σphase or a χ phase, and hence the upper limit is set to 30%. The contentis preferably in the range of from 15% to 20%.

(7) Al: 1.0% or Less:

Al is an essential element as a deoxidizing agent in steel; however, inorder to obtain the above effect, 0.005% or more of Al must be added. Anexcessive addition of Al may cause the formation of oxide baseinclusions. As a result, the surface appearance and the corrosionresistance are deteriorated, and hence the content is set to 1.0% orless. The content is preferably set in the range of from 0.01% to 0.2%.

(8) Ti: 0.05% to 0.5%, and 8≦Ti/(C+N)≦30 [In the Inequality, Ti, C, andN Represent Individual Elements Contained in Steel (Mass Percent)]:

Ti stabilizes C and N in a solid solution form as carbonitrides and Pand S as a Ti base phosphide and Ti base sulfides such as FeTiP,Ti₄C₂S₂, and TiS. Since the content of Ti has significant influences onthe size and precipitation behavior of the Ti base precipitates asdescribed above, Ti is a very important element for controlling thematerial quality in the present invention.

Since forming the precipitates as described above with various elementsdissolved in steel, Ti has effects of improving the corrosion resistanceand workability. However, when the content is less than 0.05%, C, N, P,and S cannot be formed into large and coarse Ti base precipitates andcannot be made harmless, the content must be 0.05% or more. On the otherhand, when the content is more than 0.5%, since the amount of Ti in asolid solution form is increased, hardening of steel, decrease inductility, and decrease in toughness occur, and hence the upper limit isset to 0.5%. The content is preferably in the range of from 0.10 to0.25%. In addition, since Ti forms stable carbides and nitrides with Cand N, respectively, 8≦Ti/(C+N)≦30 must be satisfied at the same time.In addition, 10≦Ti/(C+N)≦15 is preferably satisfied.

(9) N: 0.04% or Less:

When the content of N is appropriate, grain boundaries are enhanced, andhence the toughness is improved. However, when the content is more than0.04%, N precipitates in a nitride form in the grain boundaries, and thecorrosion resistance is very adversely affected. In addition, since Nforms TiN with Ti, which causes scratches on a cold-rolled sheet, inparticular, on a gloss product, the upper limit is set to 0.04%. Asdescribed above, the amount of N is preferably decreased; however, inthe case of ferrite single phase steel, ridging is effectively improvedsince the growth of columnar crystals in a slab is suppressed by TiN,and hence the content is preferably in the range of from 0.005% to 0.02%when the load required for refining is also taken into consideration.

(10) Other Components:

The composition of stainless steel manufactured according to the presentinvention basically contains the components described above. Thefollowing steel containing components besides the components describedabove may also be manufactured in accordance with the present invention;for example, there may be mentioned steel containing Fe and inevitableimpurities and steel containing optional components at contents in theranges which are not outside the scope of the present invention. Forexample, in view of improvement in grain boundary brittleness, at leastone of 0.3% or less of Ni, Cu, and Co, and 0.01% or less of B may becontained.

In addition, at least one of 0.5% or less of Nb, 0.5% or less of Zr,0.1% or less of Ca, 0.3% or less of Ta, 0.3% or less of W, 0.3% or lessof V, 0.3% or less of Sn, and 2.0% or less of Mo may be contained inview of improvement in corrosion resistance, productivity (toughnessimprovement), weldability, workability, and the like. In addition, asfor Mg, it is dissociated from slag or a refractory forming a containerfor use in a steel-manufacturing process and is contained at a contentof 0.003% or less; however, the content thereof may not cause anyserious problem.

(11) Average Diameter Dp of Ti Base Precipitates and Grain Size ofNumber Ferrite Grain:

Besides the steel component compositions described above, the presentinvention defines the average diameter Dp of grain diameters, each being[(along axis length of a Ti base precipitate+a short axis lengththereof)/2], of the Ti base precipitates in steel and the grain sizenumber of ferrite grain in a specific range. The reasons the averagediameter Dp and the grain size number of ferrite grain are focused areas follows.

In the present invention, since the P content in steel which isincreased as recycling of steel sheets is repeated is controlled in therange of from 0.01% to 0.04% (preferably 0.02% or more) by refininghaving a load equivalent to that in the past, and the sizes ofprecipitated Ti base carbides and Ti base phosphides are formed largerand coarser than a predetermined size, harmless conditions can beformed. In addition, by using a pinning effect of the Ti baseprecipitates described above, the formation of large and coarse grainsof the steel sheet is controlled, and besides the ductility and ridging,the anisotropy of mechanical properties can also be improved. In thiscase, since the precipitates such as the Ti base carbides and the Tibase phosphides have not a uniform shape, when the size is evaluated,the average diameter Dp of the Ti base precipitates in a steel sheet isused.

In the present invention, the average diameter Dp is defined as theaverage values calculated from the results of 100 precipitates which areobtained by the steps of performing electrolysis of a cross-section of atest piece in a rolling direction using a 10% AA solution (10% of acetylacetone, 1% of tetramethylammonium chloride, and methanol), sampling anextracted replica, observing 100 Ti base precipitates in a viewing fieldby a transmission electron microscope (an acceleration voltage of 200kV) at a magnification of 20,000 to 200,000, and obtaining (a long axislength of each Ti base precipitate+a short axis length thereof)/2 fromeach precipitate. When the Ti base precipitates each have an ideallyspherical form, since the long axis length is equal to the short axislength, the diameter of the precipitate may be used as the averagediameter Dp; however, in practice, the spherical form is not present inmany cases. Accordingly, as an index of the size of the Ti baseprecipitates, the largest length in the longitudinal direction isregarded as the long axis, the length in the direction perpendicularlyintersecting the center of this long axis is regarded as the short axis,and the data of (a long axis length of the Ti base precipitate+a shortaxis length thereof)/2 obtained from 100 precipitates is averaged and isdefined as the average diameter Dp (μm).

In addition, the precipitation temperatures and speeds of Ti basephosphides, Ti base carbides, and other Ti base precipitates vary inaccordance with the contents of elements forming the Ti baseprecipitates; however, when the content is increased, the precipitationtends to occur at a higher temperature and for a shorter period of time.Accordingly, box annealing is effectively carried out in which inaccordance with a component, recrystallization of a matrix andprecipitation of Ti base precipitates are optionally performed at atemperature close to the precipitation nose temperature.

(12) Average Diameter Dp [(Long Axis Length of Ti Base Precipitate+ShortAxis Length Thereof)/2] of Ti Base Precipitates of Hot-Rolled AnnealedSteel Sheet and Cold-Rolled Annealed Steel Sheet: 0.05 μm to 1.0 μm:

In general, Ti base precipitates in a steel sheet have been known asmaterials degrading the workability thereof. However, in the hot-rolledannealed steel sheet and cold-rolled sheet of the present invention,when the Ti base precipitates are grown in a large and coarse form tohave an average diameter Dp of 0.05 μm to 1.0 μm, inversely, the Ti baseprecipitates are made harmless. Furthermore, the matrix is purified, andsuperior workability of the steel sheet can be obtained. In addition,when a hot-rolled annealed steel sheet having an average diameter Dp of0.05 μm to 1.0 μm is further processed by cold rolling, since theamounts of C and P dissolved in the hot-rolled steel sheet are decreasedin addition to the decrease in recrystallization temperature, {111}textures parallel to a sheet surface, which advantageously improve the rvalue, are significantly grown. Accordingly, the average diameter Dp ofthe Ti base precipitates is one of the most important points of thepresent invention.

In addition, since the recrystallization temperature is decreased, theintermediate annealing temperature or the final annealing temperature isalso decreased. As a result, since the amounts of C and P dissolved inthe final cold-rolled steel sheet are decreased, softening, highductility, and a low YS of steel can be achieved. However, when Ti baseprecipitates are very fine having an average diameter Dp of less than0.05 μm, since the thermal stability of the Ti base precipitates isdegraded due to strain caused by cold rolling, the Ti base precipitatesare redissolved in annealing of a cold-rolled steel sheet, and as aresult, in addition to the increase of P and C in a solid solution form,the steel is hardened by a precipitation effect caused by the fine Tibase precipitates. Furthermore, the growth of the {111} textures issuppressed by the fine precipitates, and as a result, the materialquality is degraded. Accordingly, the lower limit of the averagediameter Dp of the Ti base precipitates is set to 0.05 m. The Ti baseprecipitates having a larger size within the range described above areeffective; however, when the average diameter Dp is more than 1.0 μm,although the ductility is effectively improved, the r value is rapidlydecreased. The reason for this has been believed that since abnormalstructure are formed around the large and coarse precipitates in coldrolling, {110} recrystallization texture is liable to be formed which isharmful to the r value. According to the reason described above, theaverage diameter Dp of the Ti base precipitates in hot-rolled annealedand cold-rolled annealed steel sheets is set in the range of from 0.05μm to 1.0 μm, preferably from 0.2 μm to 0.6 μm, and more preferably from0.3 μm to 0.5 μm.

(13) Grain Size Number of Ferrite Grain of Hot-Rolled Annealed SteelSheet and Cold-Rolled Annealed Steel Sheet: 6.0 or More:

The grain size number of a hot-rolled annealed steel sheet influencesthe ridging and the r value of a cold-rolled annealed steel sheet. Sincethe number of grain boundaries functioning as nucleus-generating sitesfor recrystallization is increased as the crystal grain size is smaller,the degree of integration of the {111} texture in a final annealed steelsheet is increased, and hence it is advantageous for the r value. Asdescribed above, a good correlation is present between the crystal grainsize of the hot-rolled steel sheet and the r value of the cold-rolledsteel sheet, and the r value is improved as the crystal grains of thehot-rolled annealed steel sheet become larger and coarser; however, whenthe grain size number is more than 6.0, the ridging and the anisotropyof mechanical properties are increased, and when the grain sizes becomefurther larger and coarser, the r value is decreased. By the reasonsdescribed above, the lower limit of the grain size number of ferritegrain of the hot-rolled annealed steel sheet is set to 6.0. In the caseof an intermediate-annealed steel sheet which is processed by threetimes annealing including intermediate annealing and two-time coldrolling, since the recrystallization temperature of theintermediate-annealed steel sheet is decreased as compared to that of ahot-rolled steel sheet, the grain size number is preferably set to 6.5or more. In the present invention, the grain size number is measured bya section method in accordance with JIS G0552 (Methods of grain sizenumber of ferrite grain determination test for steel) in which fiveviewing fields on a cross-section surface in the rolling direction (Ldirection) are observed at a magnification of 100, and the grain sizesnumber thus measured are then averaged to obtain the average value.

Even when a steel sheet is manufactured through cold rolling and finalannealing, the grain size number of ferrite grain of a final-annealedsteel sheet must be 6.0 or more. The ferrite crystal grain size of thefinal-annealed steel sheet (ferrite grain size after final annealing)influences the surface roughness thereof after forming processing. Whenthe grain size is increased, the ductility and the r value can beimproved; however, when the grain size number is less than 6.0, as thegrain diameter becomes larger and coarser, a rough surface, a so-calledorange peel, is formed on a product surface after processing, and as aresult, in addition to deterioration of the appearance, the corrosionresistant and the formability are degraded resulting from the roughsurface. Hence, the grain size number of the final-annealed steel sheetmust be 6.0 or more and preferably 6.5 or more.

(14) Precipitation Ratio of Ti and P in Hot-Rolled Annealed Steel Sheetand Cold-Rolled Annealed Steel Sheet:

When at least 50% of the total Ti content in a hot-rolled annealed steelsheet and a cold-rolled annealed steel sheet is precipitated in the formof the Ti base precipitates, almost all the P and C in steel can beprecipitated in the form of the Ti base precipitates. Accordingly, largeamounts of the P and C dissolved in steel can be decreased. When lessthan 50% of the total Ti content is precipitated in the form of the Tibase precipitates, in addition to insufficient reduction of the P and Cdissolved in steel, fine precipitates are increasingly precipitated, andas a result, the effect of improving workability cannot be obtained.

More preferably, it is desirable that at least 70% of the total Ticontent in the hot-rolled annealed steel sheet and the cold-rolledannealed steel sheet be precipitated. Even more preferably, it isdesirable that in addition to the precipitated amount of Ti describedabove, the amount of P base precipitates be at least 50% of the total Pcontent.

The ratio of a part of the total Ti content in each of the hot-rolledannealed steel sheet and the cold-rolled annealed steel sheet, which wasprecipitated in the form of the Ti base precipitates, was obtained bymultiplying 100 and an analyzed amount (mass percent) of theprecipitated Ti in steel divided by the total Ti content (mass percent)therein. “The total Ti amount (mass percent)” was measured in accordancewith JIS G1258:1999 (Iron and steel—Methods for inductively coupledplasma atomic emission spectrometry). That is, a sample is dissolved inan acid (hydrochloric acid+nitric acid). After a residue is recovered byfiltration and is processed by an alkaline fusion (sodiumcarbonate+sodium borate), the residue thus processed is dissolved inhydrochloric acid and is mixed together with the acid solution mentionedabove, and the mixture thus obtained is diluted with purified water to apredetermined volume. Subsequently, by an ICP emission spectrometer, theTi amount (TiA) in this solution is quantified.Total Ti amount (mass percent)=TiA/sample weight×100

“The precipitated Ti amount (mass percent)” is obtained byconstant-current electrolysis (current density of 20 mA/cm² or less) ofa sample using an acetyl acetone base electrolyte (a so-called AAsolution). A residue in the electrolyte after this electrolysis isrecovered by filtration and is processed by an alkaline fusion (sodiumperoxide+lithium methaborate), and then the residue thus processed isdissolved by acid and is diluted with purified water to a predeterminedvolume. Subsequently, by an ICP emission spectrometer, the Ti amount(TiB) in this solution is quantified.Precipitated Ti amount (mass percent)=TiB/sample weight×100

In addition, the ratio of a part of the total P content in each of thehot-rolled annealed steel sheet and the cold-rolled annealed steelsheet, which was precipitated in the form of the Ti base precipitates,was obtained by multiplying 100 and an analyzed amount (mass percent) ofthe precipitated P in steel divided by the total P content (masspercent) therein. “The total P amount (mass percent)” was measured inaccordance with JIS G1214:1998 (Iron and steel_|Methods fordetermination of phosphorus content) That is, a sample is dissolved inan acid (nitric acid+hydrochloric acid+perchloric acid) and white fumetreatment is performed using perchloric acid so as to formorthophosphoric acid from phosphorus, followed by the formation of acomplex with molybdic acid. Subsequently, by molybdophosphoric acid-bluecomplex (molybdenum blue) absorption spectroscopy, the P amount (PA) inthis solution is quantified.

Total P amount (mass percent)=PA/sample weight×100

On the other hand, “the precipitated P amount (mass percent)” isobtained by constant-current electrolysis (current density of 20 mA/cm²or less) of a sample using an acetyl acetone base electrolyte (aso-called AA solution). A residue in the electrolyte after thiselectrolysis is recovered by filtration and is dissolved by acid (nitricacid+hydrochloric acid+perchloric acid), and phosphorus is thenprocessed by white fume treatment using perchloric acid to formorthophosphoric acid from phosphorus, followed by the formation of acomplex with molybdic acid. Subsequently, by molybdophosphoric acid blue(molybdenum blue) absorption spectroscopy, the P amount (PB) in thissolution is quantified.Precipitated P amount (mass percent)=PB/sample weight×100(15) Method for Manufacturing Ti-Containing Ferritic Stainless SteelSheet Having Low Yield Strength

Next, a preferable method for manufacturing the Ti-containing ferriticstainless steel sheet having a low yield strength according to thepresent invention will be described.

A process for manufacturing the stainless steel sheet of the presentinvention includes a steel-manufacturing step, a step of manufacturing aslab from molten steel by continuous casting or the like, a step ofheating the slab, a hot rolling step, and a step of annealing ahot-rolled steel sheet. Alternatively, in addition to the stepsdescribed above, a cold-rolled steel sheet is manufactured by a seriesof steps including a cold rolling step and a final annealing step. Inthe present invention, the conditions of the annealing step of thehot-rolled steel sheet after hot rolling and of the final annealing stepafter cold rolling are defined.

According to the present invention, after hot rolling, recrystallizationannealing is first performed so that the average diameter Dp of the Tibase precipitates is in a specific range. Specifically, the Ti baseprecipitates indicate a phosphide (FeTiP), carbides (TiC, TiS, andTi₄C₂S₂), and the like. In many cases, the precipitates are mostlycomposed of FeTiP and TiC having a precipitation nose temperature T ofapproximately 650° C. to 850° C.

(16) Annealing of Hot-Rolled Steel Sheet:

In the present invention, it is important that the Ti base precipitatesin the hot-rolled steel sheet be grown large and coarse to have apredetermined size. As the methods therefor, for example, control of hotrolling and a coiling temperature, or box annealing (box furnace)performed longer than continuous annealing may be applied. Regardless ofthe methods, it is important that C and P dissolved in the hot-rolledsteel sheet be precipitated in the form of large and coarse Ti baseprecipitates having an average diameter Dp of 0.05 μm to 1.0 μm so as tobe made harmless. Accordingly, the workability of steel is improved.Since the optimum temperature is in the vicinity of the precipitationnose temperatures of FeTiP and TiC, it is naturally understood that Ti,P, C, S, and N in the steel and the coiling condition for a hot-rolledsheet have influences on the optimum temperature. However, the annealingtemperature and a soaking temperature are preferably in the range offrom 650° C. to 850° C. in which the precipitation is most effectivelypromoted. A holding time of box annealing, the hot rolling conditions,and a holding time or a cooling rate in a coiling or a cooling step areset so that the average diameter Dp of the Ti base precipitates iscontrolled in the range described above. Furthermore, at least 50% ofthe total Ti content in the steel is precipitated in the form of the Tibase precipitates. A preferable holding time is 1 to 100 hours inconsideration of practical operation and is more preferably in the rangeof from 1 to 10 hours.

In manufacturing of the stainless steel sheet of the present invention,the form of the precipitates in the hot-rolled annealed steel sheetdetermines the properties of the steel, and when the Ti baseprecipitates are grown larger and coarser than a predetermined size, amatrix of the hot-rolled annealed steel sheet can be purified, and therecrystallization temperature after cold rolling is decreased. Inaddition, since the amounts of C and P dissolved in the hot-rolledannealed steel sheet are decreased, and the growth of the {111}textures, which effectively improve the r value, is significantlypromoted, and the r value in the final cold-rolled steel sheet is alsoimproved. By the decrease in annealing temperature of a cold-rolledsteel sheet, which will be described later, redissolution of C and Pprecipitated in the form of the Ti base precipitates is suppressed, andas a result, a low yield strength, softening, and high ductility of thefinal cold-rolled steel sheet can be achieved.

The annealing temperature for the hot-rolled steel sheet must becontrolled in the range of (a precipitation nose temperature of Ti baseprecipitates±50° C.). Otherwise, the Ti base precipitates having apredetermined average diameter Dp cannot be precipitated. In addition,at least 50% of Ti in the steel sheet cannot be precipitated in the formof the Ti base precipitates. Accordingly, the TTP curve was formed fromthe precipitation behavior of Ti, and as a result, the precipitationnose temperature T was found. Particular methods for forming the TPPcurve and for obtaining the precipitation nose temperature T are thesame as those described with reference to FIG. 4. That is, for eachsteel having the corresponding composition, precipitated Ti amounts weremeasured at various annealing temperatures (500° C. to 1,000° C. atregular intervals of 25° C.) and for various annealing times (1 minute,10 minutes, 1 hour, and 100 hours), and a precipitation curve wasobtained in which the precipitated Ti amount was at least 50% of thetotal Ti amount in a steel sheet. Subsequently, the temperaturecorresponding to the nose portion N shown in FIG. 4 was regarded as theprecipitation nose temperature T (° C.) of the Ti base precipitates(carbides, phosphides, and the like).

Since an object of annealing of the hot-rolled steel sheet includesrecrystallization of ferrite structures thereof, the annealingtemperature and the annealing time are set to (a precipitation nosetemperature of Ti±50° C.) so that Ti base precipitates having apredetermined size and a predetermined precipitated amount of (at least50% of the total Ti amount in steel) is obtained in a short period oftime. When the annealing temperature is too high, although therecrystallization occurs, the size of the Ti base precipitates is smalland the amount thereof is small, and as a result, large amounts of C andP in a solid solution form are allowed to remain in the matrix. Inaddition, when the annealing temperature is low, the recrystallizationis unlikely to occur, and a small amount of the Ti base precipitates isonly precipitated. In determining the annealing temperature, it iseffective to estimate the precipitation nose of the Ti base precipitatesfrom the precipitated amount thereof with reference to results obtainedfrom investigation performed beforehand.

(17) Final Annealing:

Recrystallization annealing (final annealing) is performed for thecold-rolled steel sheet at a temperature less than (a precipitation nosetemperature T of Ti base precipitates+100° C.) so that the grain sizenumber of ferrite grain is 6.0 or more.

As the final annealing is performed at a higher temperature, {111}orientation grains are selectively grown, and a high r value can beobtained. When the final annealing temperature is low, andnon-recrystallized structures remain, the workability is degraded. Inorder to increase the r value, final annealing performed at a hightemperature is effective; however, on the other hand, the crystal grainsize is increased, and a rough surface is formed after forming, therebycausing decrease in formability limit and degradation of corrosionresistance. Accordingly, the final annealing temperature is preferablyincreased as long as a grain size number of 6.0 or more and preferablyof 6.5 or more can be ensured. In addition, in particular, the presentinvention is characterized in that P and C are precipitated in the formof large and coarse phosphides such as FeTiP and carbides such as TiC,respectively, so as to be harmless. However, the Ti base precipitatesmentioned above tend to be dissolved at a temperature of 850° C. ormore. For example, even in continuous annealing in which heating israpidly performed and is held for a short period of time, when heattreatment is performed at a temperature of more than 900° C.,dissolution of the precipitates proceeds, and hence the upper limit of apreferable temperature is set to 900° C. In addition, although the lowerlimit of the final annealing temperature is the recrystallizationtemperature, a preferable temperature is set so that the grain sizenumber is in the range of from 6.0 to 7.5. Furthermore, more preferably,the temperature is set so that the grain size number is in the range offrom 6.5 to 7.0.

The grain size number of the cold-rolled annealed steel sheet influencesthe ridging, r value, YS, and workability. By annealing at a hightemperature, the crystal grain size is increased, and by agrain-diameter effect, the YS is decreased (Holl-pitch rule), and theductility is improved. However, when the crystal grain number is lessthan 6.0, rough surfaces are apparently formed, and in addition toincrease in anisotropy of the mechanical properties, the appearance isdeteriorated. In addition, due to the rough surfaces, the corrosionresistance and the workability are degraded. In addition, when theannealing temperature for the cold-rolled steel sheet is higher than theprecipitation nose temperature T of Ti by more than 100° C., the Ti baseprecipitates are redissolved, and the YS is increased.

In the case of a hot-rolled annealed steel sheet in which the Ti baseprecipitates are grown larger and coarser than a certain size, thelarger and coarser precipitates remain after final annealing isperformed, and a cold-rolled annealed steel sheet can be obtained whichis made of fine grains and which has a low yield strength.

Steel slabs having the compositions shown in Table 1 were heated andthen hot-rolled, thereby forming hot-rolled steel sheets having athickness of 4 mm. For each of the hot-rolled steel sheets, precipitatedTi amounts were measured at various annealing temperatures (500° C. to1,000° C. at regular intervals of 25° C.) and for various annealingtimes (1 minute, 10 minutes, 1 our, and 100 hours), and the range inwhich the precipitated Ti amount was at least 50% of the total Ticontent in the steel sheet was obtained, thereby forming a TTP curve(precipitation start curve) of the Ti base precipitates as shown in FIG.4. Subsequently, the precipitation nose temperature T (770° C.) wasdetermined. Next, recrystallization annealing was performed for thehot-rolled steel sheet at 800° C. (the precipitation nose temperatureT±50° C.) so as to change the size of the Ti base precipitates, andhot-rolled annealed steel sheets having average grain diameters Dp of0.03 μm and 0.28 μm were obtained. Subsequently, after a cold-rolledsteel sheet having a thickness of 0.8 mm was formed by cold rolling at atotal reduction in thickness of 80%, annealing of the cold-rolled steelsheet was performed for various periods of time, thereby formingcold-rolled annealed steel sheets having various grain sizes. Next, thecrystal grain size of the hot-rolled annealed steel sheet and the yieldstrength of the cold-rolled annealed steel sheet were compared to eachother. The results are shown in Table 2.

In the present invention, the yield strength is measured in accordancewith JIS Z2241.

The average diameter Dp of the Ti base precipitates in the hot-rolledsteel sheet of each of sample Nos. A to E is set to 0.28 μm, and theaverage diameter Dp of the Ti base precipitates in the hot-rolled steelsheet of each of sample Nos. F to J is set to 0.03 μm. The relationshipbetween the grain size number of ferrite crystal grains of thehot-rolled annealed steel sheet and the yield strength of thecold-rolled annealed steel sheet is shown in FIG. 3. From Table 2 orFIG. 3, it was found that even among steel materials having the samecomponent system, between cold-rolled steel sheets having the samegrains size, a lower yield strength can be obtained from one of thecold-rolled steel sheets which is made from a hot-rolled annealed steelsheet having a larger average diameter Dp of the Ti base precipitates.

In addition, it was also found that when the average diameter Dp of theTi base precipitates of the hot-rolled annealed steel sheet is set inthe range of from 0.05 μm to 1.0 μm, a preferably low yield strength isobtained. In addition, it was also found that when a cold-rolledannealed steel sheet is processed by deep drawing which has a grain sizenumber of 6.0 or more and preferably 6.5 or more and which is obtainedby annealing at a precipitation nose temperature T of Ti baseprecipitates+100° C. or less, rough surfaces are not formed, and that,in addition, the Ti base precipitates in the cold-rolled steel sheet arenot dissolved. As the lower limit of the final annealing temperature, atemperature is preferably set so that the grain size described above issatisfied and that non-recrystallized grains are not allowed to remain.In addition, in order to precipitate Ti base carbides and Ti basephosphides so that the shapes thereof are as large and coarse aspossible, the annealing temperature of the cold-rolled steel sheet ispreferably set to a precipitation nose temperature T of Ti baseprecipitates+50° C. or less.

The grain diameters described in the present invention are all measuredby a section method in accordance with JIS G0552 in which five viewingfields on a cross-section surface in the rolling direction (L direction)are observed at a magnification of 100, and the grain sizes thusmeasured are then averaged to obtain the average value.

In the present invention, as for steps other than the annealing step forthe hot-rolled steel sheet after hot rolling and the annealing step forthe cold-rolled steel sheet after cold rolling, the conditions thereofare not specifically limited; however, in the individual steps, thefollowing conditions are preferable.

(18) Slab Heating:

When a slab heating temperature is too low, rough surfaces are formed,and in addition, it becomes difficult to perform hot rolling underpredetermined conditions by rough rolling. On the other hand, when theslab heating temperature is too high, the structure of a hot-rolledsteel sheet becomes large and coarse, and structures become non-uniformin the thickness direction. In addition, Ti₄C₂S₂ is redissolved, and Cand S are dissolved in the steel in a solid solution form. Accordingly,the slab heating temperature is set in the range of from 950° C. to1,150° C. The preferable temperature range is in the range of from1,000° C. to 1,100° C.

(19) Hot Rough Rolling:

At least one pass of hot rough rolling (hereinafter simply referred toas rough rolling) is performed at a rolling temperature of 850° C. to1,100° C. and at a reduction in thickness of 40% or more per pass. Whenthe rolling temperature of rough rolling is less than 850° C.,recrystallization is unlikely to occur, the workability of afinal-annealed steel sheet is inferior, and in-plane anisotropy isincreased. In addition to those described above, a load applied ontorolling rolls is increased, and as a result, the serviceable lifethereof is decreased. On the other hand, when the temperature is morethan 1,100° C., a structure is formed in which the ferrite grains extendin the rolling direction, and as a result, the anisotropy is increased.Hence, the rolling temperature in rough rolling is set in the range offrom 850° C. to 1,100° C. The preferable temperature range is in therange of from 850° C. to 1,000° C.

In addition, when the reduction in thickness in rough rolling is lessthan 40% per pass, since a large amount of a non-crystallized part in aband shape remains at a central portion in the thickness direction,ridging is generated in the cold-rolled steel sheet, and hence theworkability thereof is degraded. However, when the reduction inthickness in rough rolling is more than 60% per pass, seizing may occurin rolling and biting defects may also occur in some cases. Accordingly,in particular, the reduction in thickness is preferably in the range offrom 40% to 60% per pass. In addition, in a steel material having a lowhigh-temperature strength, an intensive shear strain may be generated onsurfaces of the steel sheet in rough rolling so that anon-recrystallized structure remains at the central portion in thethickness direction, and in addition, seizing may also occur in roughrolling. In the case described above, whenever necessary, lubricationtreatment may be performed so as to have a friction coefficient of 0.3or less. When rough rolling is performed at least one pass under theconditions in which the rolling temperature and the reduction inthickness described above are satisfied, deep drawing properties can beimproved. This one pass may be performed at any stage in rough rolling;however, in consideration of a rolling machine capacity, this pass ismost preferably performed as the last pass.

(20) Hot Final Rolling:

At least one pass of hot final rolling (hereinafter simply referred toas final rolling) following the rough rolling is preferably performed ata rolling temperature of 650° C. to 900° C. and at a reduction inthickness of 20% to 40% per pass. When the rolling temperature is lessthan 650° C., deformation resistance is increased, a reduction inthickness of 20% or more per pass is difficult to reliably obtain, andin addition, the load applied onto rolls is increased. On the otherhand, when the final rolling temperature is more than 900° C., theaccumulation of rolling strain is decreased, and an effect of improvingthe workability in a subsequent step is decreased. Hence, the finalrolling temperature is set in the range of from 650° C. to 900° C. andpreferably in the range of from 700° C. to 800° C.

In addition, in the final rolling, when the reduction in thickness at arolling temperature of 650° C. to 900° C. is less than 20%, {100}//ND, alarge amount of {100}//ND colony remains which causes decrease in rvalue and generation of ridging. In the present invention, the {100}//NDmeans that an <100> orientation vector of a crystal is parallel to anorientation vector (ND orientation) perpendicular to the rollingsurface. In addition, the {100}//ND colony is an assembly of adjacentcrystals in which the angle formed of each <100> orientation vector withthe orientation vector (ND orientation) perpendicular to the rollingsurface is within 30°. On the other hand, when the reduction inthickness is more than 40%, biting defects and shape defects may occur,and as a result, steel surface properties may be deteriorated.Accordingly, in the final rolling, at least one pass of rolling at areduction in thickness of 20% to 40% is performed. The preferable rangeis 25% to 35%. When the final rolling is performed at least one passunder the conditions in which the rolling temperature and the reductionin thickness described above are satisfied, deep drawing properties canbe improved. This one pass may be performed at any stage; however, inconsideration of a rolling machine capacity, this pass is mostpreferably performed as the last pass.

(21) Cold Rolling:

As described above, after the annealed steel sheet processed byannealing for the hot-rolled steel sheet is cold-rolled,recrystallization annealing is further performed. The conditions forcold rolling are not specifically limited, and a general method may beused.

Cold rolling may be carried out at least twice whenever necessary withintermediate annealing which is performed therebetween at a temperatureof 600° C. to 900° C. In this case, the total reduction in thickness ora reduction ratio represented by (reduction in thickness of first coldrolling/reduction in thickness of final cold rolling) is preferably setto 75% or more and 0.7 to 1.3, respectively. In addition, grain sizenumber of ferrite grain right before the final cold rolling is set topreferably 6.0 or more, more preferably 6.5 or more, and even morepreferably 7.0 or more. When the intermediate annealing temperature isless than 600° C., recrystallization insufficiently occurs, and inaddition to decrease in r value, the ridging apparently occurs due tonon-recrystallized band-shaped structure. On the other hand, when theintermediate annealing temperature is more than 900° C., anintermediate-annealed steel sheet structure becomes large and coarse, Tibase carbides and Ti base phosphides are redissolved, and as a result,the Ti base precipitates cannot be maintained to have a predeterminedsize. Furthermore, C and P in a solid solution form are increased insteel, and hence the formation of structures having suitable deepdrawing properties is interfered with. The increase in total reductionin thickness has an influence on the improvement in development of the{111} textures of the final-annealed steel sheet, and in addition, the rvalue is advantageously improved.

Furthermore, in cold rolling in the present invention, it is preferablethat by using a tandem rolling machine, the cold rolling is preferablyperformed in one direction with a work roll having a roll diameter of300 mm or more. In order to suppress shear deformation of a material tobe rolled and to increase (222)/(200) for improving the r value, theinfluences of the roll diameter and rolling direction are preferablytaken into consideration. In general, in the final cold rolling forstainless steel, in order to obtain surface gloss, a work roll having asmall roll diameter, such as 200 mm diameter or less, has been used;however, according to the present invention, since it is particularlyintended to improve the r value, even in the final cold rolling, a workroll having a large roll diameter of 300 mm or more is preferably used.

That is, compared to reverse rolling using a roll having a diameter of100 to 200 mm, when tandem rolling is used in which rolling is performedin one direction using a roll having a diameter of 300 mm or more, theshear deformation on surfaces is decreased, and the r value isadvantageously improved. Since the large diameter roll is used as a workroll for rolling, and in addition, one direction rolling (tandemrolling) is performed, the (222) is increased. In order to stably obtaina higher r value, a line pressure (rolling load/sheet width) must beincreased so that a strain is uniformly applied in the thicknessdirection, and hence it is effective that decrease in hot rollingtemperature, higher alloying, increase in hot rolling speed beoptionally combined with each other.

According to the present invention, as described above, P is allowed toremain at a content of from 0.01% to 0.04% in steel, the P beingparticularly likely to be contained in starting materials used for steelmanufacturing, so as to be precipitated in the form of Ti baseprecipitates having a predetermined size. Hence, the precipitates aremade harmless, and suppression of grain growth by an appropriate pinningeffect of the precipitates and higher purification of the matrix can beachieved. As a result, compared to steel in which purification isperformed simply by refining so as to form fine precipitates or so as tosuppress the precipitation itself, steel having fine grains and a lowyield strength can be obtained. According to the present invention, aferritic stainless steel sheet having a low yield strength can bemanufactured in which the ductility, ridging, and anisotropy ofmechanical properties are also improved.

When a pipe is formed by welding using the steel sheet of the presentinvention described above, welding methods are not particularly limited,and for example, general arc welding methods such as MIG (Metal InertGas), MAG (Metal Active Gas), and TIG (Tungsten Inert Gas); resistancewelding methods such as spot welding and seam welding; high-frequencyresistance welding such as electric resistance welding; andhigh-frequency induction welding may be used.

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to examples.

EXAMPLE 1 Tables 3 and 4

Steel formed from steel slabs 1 to 4 having compositions (balance beingsubstantially Fe) including P and the like shown in Table 3 washot-rolled under the following conditions (a slab heating temperature of1,100° C., a rough rolling temperature of 990° C., a reduction inthickness of rough rolling of 35%, a final rolling temperature of 752°C., and a reduction in thickness of final rolling of 30%), followed byannealing of the hot-rolled steel sheet under the following conditions(a box annealing temperature of 780° C., a holding time for boxannealing of 10 hours, an intermediate annealing temperature of 850° C.,a total reduction in thickness of 85%, a reduction ratio of 1.0, and afinal annealing temperature of 900° C., thereby forming hot-rolled steelsheets. In addition, as for the steel 3, in a rolling step in which thethickness was further gradually decreased to 5 mm, 2.3 mm, and 0.8 mm,three times annealing including intermediate annealing, cold rolling bya two-time cold rolling method, and final rolling were performed. Inaddition, for each of the precipitation nose temperatures T of Tiprecipitates of the steel slabs 1 to 4 in Table 3, precipitated Tiamounts were measured at various annealing temperatures (500° C. to1,000° C. at regular intervals of 25° C.) and for various annealingtimes (1 minute, 10 minutes, 1 hour, and 100 hours), and a precipitationcurve was obtained in which the precipitated Ti amount was at least 50%of the total Ti content in the steel sheet. In addition, the temperatureT corresponding to the nose portion N shown in FIG. 4 was defined as aprecipitation nose temperature T (° C.) of the Ti base precipitates(carbides, phosphides, and the like). The precipitation nosetemperatures T thus obtained are shown in Table 3.

The properties of the hot-rolled steel sheets and the cold-rolled steelsheets were investigated. The results are shown in Table 4. The grainsize numbers of ferrite grains of the hot-rolled steel sheet and thefinal-annealed steel sheet were measured on a cross-section in therolling direction (L direction) by a section method in accordance withJIS G0552. In addition, by using a test piece JIS No. 13-B, YS, TS, andEl. of the hot-rolled annealed steel sheets and the cold-rolled annealedsteel sheets were measured. In addition, a mono-axial tensile stress of15% was applied beforehand, and the r values (rL, rD, rC) in individualdirections were obtained in accordance with the three point method.Subsequently, the average r value and Δr were calculated by thefollowing equations, and the average values were obtained when thenumber of data points n was 3.Average r=(rL+2rD+rC)/4,Δr=(rL−2rD+rC)/2.

(Where rL, rD and rC represent, respectively, r values in the rollingdirection, in a direction of 45° with respect to the rolling direction,and in a direction of 90° with respect to the rolling direction.)

In addition, an undulation height of a steel sheet surface, whichindicated the resistance to generation of rough surface, was measured bythe steps of forming a test piece JIS No. 5 by cutting the steel sheetalong the rolling direction, processing the test piece by #800 wetpolishing, applying a tensile strain of 25%, and measuring the roughnessgenerated on the surface along a length of 1 cm in the directionperpendicular to the tensile direction using a stylus method, and theevaluation was performed using the value (Ry) of the surface roughness.In this measurement, 5 points were measured in the range of ±10 mm fromthe center of the test piece in the longitudinal direction at regularintervals of 5 mm in the longitudinal direction, and up to 10 data ofthe average roughness were obtained.

The ridging resistance was measured by the steps of forming a test pieceJIS No. 5 by cutting the steel sheet along the rolling direction,processing two surfaces of the test piece by a wet polishing paper of#600, applying a tensile strain of 25%, and measuring undulation heightsof the center of the test piece in the tensile direction and in thedirection perpendicular thereto using a surface roughness meter, and theundulation heights thus measured were categorized into the followingfive ranks A to E for evaluation. The rank A indicates an undulationheight of 15 μm or less, the rank B indicates an undulation height of 30μm or less, the rank C indicates an undulation height of 45 μm or less,the rank D indicates an undulation height of 60 μm or less, and the rankE indicates an undulation height of more than 60 μm.

When the ridging is categorized into the ranks C, D, and E, although ther value and the ductility are improved, due to the irregularities of theridging, the decrease in workability limit occurs; hence, the ranks Aand B are regarded as an acceptable level. In addition, the loadrequired for refining was evaluated based on the time required forrefining. In this evaluation, a refining time required for reducing theP content in molten steel to 0.015% is regarded as the standard, inwhich recycling of scrap, dust, and slag is not performed; the case inwhich the refining time is 150% or more of the standard time iscategorized as non-acceptable level C; the case in which the refiningtime is more than 70% to less than 150% is categorized as acceptablelevel B; and the case in which the refining time is decreased to 70% orless is categorized as acceptable level A. When dust and slag generatedin refining are recycled, the P amount contained into molten steel isincreased, and as a result, the refining load is increased.

The ratio of precipitation in the form of the Ti base precipitates tothe total Ti content in each of the hot-rolled annealed steel sheet andthe cold-rolled annealed steel sheet was obtained by multiplying 100 andan analyzed amount (mass percent) of a precipitated Ti in steel dividedby the total Ti content (mass percent) therein. “The total Ti amount(mass percent)” was measured in accordance with JIS G1258:1999 (Iron andsteel-Methods for inductively coupled plasma atomic emissionspectrometry). That is, a sample is dissolved in an acid (hydrochloricacid+nitric acid). After a residue is recovered by filtration and isprocessed by an alkaline fusion (sodium carbonate+sodium borate), theresidue thus processed is dissolved in hydrochloric acid and is mixedtogether with the acid solution mentioned above, and the mixture thusobtained is diluted with purified water to a predetermined volume.Subsequently, by an ICP emission spectrometer, the Ti amount (TiA) inthis solution is quantified.Total Ti amount (mass percent)=TiA/sample weight×100

“The precipitated Ti amount (mass percent)” is obtained byconstant-current electrolysis (current density of 20 mA/cm² or less) ofa sample using an acetyl acetone base electrolyte (a so-called AAsolution). A residue in the electrolyte after this electrolysis isrecovered by filtration and is processed by an alkaline fusion (sodiumperoxide+lithium methaborate), and then the residue thus processed isdissolved by acid and is diluted with purified water to a predeterminedvolume. Subsequently, by an ICP emission spectrometer, the Ti amount(TiB) in this solution is quantified.Precipitated Ti amount (mass percent)=TiB/sample weight×100

In addition, the ratio of precipitation in the form of the Ti baseprecipitates to the total P content in each of the hot-rolled annealedsteel sheet and the cold-rolled annealed steel sheet was obtained bymultiplying 100 and an analyzed amount (mass percent) of a precipitatedP in steel divided by the total P content (mass percent) therein. “Thetotal P amount (mass percent)” was quantitatively measured in accordancewith JIS G1214:1998 (Iron and steel Methods for determination ofphosphorus content). That is, a sample is dissolved in an acid (nitricacid+hydrochloric acid+perchloric acid), and phosphorus is thenprocessed by white fume treatment using perchloric acid to formorthophosphoric acid, followed by the formation of a complex withmolybdic acid. Subsequently, by molybdophosphoric acid-blue complex(molybdenum blue) absorption spectroscopy, the P amount (PA) in thissolution is quantified.Total P amount (mass percent)=PA/sample weight×100

On the other hand, “the precipitated P amount (mass percent)” isobtained by constant-current electrolysis (current density of 20 mA/cm²or less) of a sample using an acetyl acetone base electrolyte (aso-called AA solution). A residue in the electrolyte after thiselectrolysis is recovered by filtration and is dissolved in an acid(nitric acid+hydrochloric acid+perchloric acid), and then phosphorus isprocessed by white fume treatment using perchloric acid to formorthophosphoric acid, followed by the formation of a complex withmolybdic acid. Subsequently, by molybdophosphoric acid blue (molybdenumblue) absorption spectroscopy, the P amount (PB) in this solution isquantified.Precipitated P amount (mass percent)=PB/sample weight×100

The results are shown in Table 4. In FIG. 1, as for Nos. 5 to 10, therelationship among the average diameter Dp of the Ti base precipitates,the average r value, and the ductility El. is shown. In addition, inFIG. 2, as for Nos. 15 to 19, the relationship among the averagediameter Dp of the Ti base precipitates, the Δr value (anisotropy), andthe surface roughness is shown. From FIG. 1, in the relationship betweenthe average diameter Dp of the precipitates and the average r value, itwas understood that the maximum value is obtained at a Dp ofapproximately 0.03 μm, and that the Dp is effectively controlled in therange of from 0.05 μm to 1.0 μm so as to obtain an average r value of1.1 or more of the hot-rolled steel sheet. FIG. 2 shows the influencesof the grain size number of the cold-rolled annealed steel sheet on thesurface roughness and the Δr thereof by way of example. It wasunderstood that when the grain size number of the cold-rolled annealedsteel sheet is 6.0 or less, the surface roughness is drasticallyincreased, and in addition, that the anisotropy (Δr) of the r value isalso increased.

Hereinafter, the results shown in Table 4 will be described.

No. 1 is a comparative example in which the refining time was short. Inthis comparative example, the p content was not sufficiently reduced byrefining, such as 0.046%; hence, the ductility El. and the average rvalue were low, and the YS and TS were high.

Nos. 2 and 3 are examples in which P was decreased to 0.04% or less. Inthe examples of the present invention, since the P was decreased, theductility El. and the average r value were high, and the YS and TS werelow.

No. 4 is an example in which P was decreased to 0.008%. In thiscomparative example, although the properties of the steel were improved,the time required for refining was long.

No. 5 is a comparative example in which the average diameter Dp of theTi base precipitates was small, such as 0.03 μm, the YS was high, theaverage r value was low, and the workability was not good.

Nos. 6 to 9 are examples in which the average diameter Dp of the Ti baseprecipitates was grown larger and coarser, such as 0.07 μm to 0.88 μm,and in which the hot-rolled steel sheets were formed uniformly so thatthe grain size number were the same, such as 6.1. These examples of thepresent invention show that, compared to the result of No. 5, theworkability (YS was low, and elongation was high) was improved as theaverage diameter Dp of the Ti base precipitates was increased in therange described above.

No. 10 is a comparative example in which since the average diameter Dpof the Ti base precipitates was 1.15 μm, which was more than an upperlimit of 1.0 μm according to the present invention, the average r valuewas decreased.

Nos. 11 and 12 are comparative examples in which since the grain size ofthe hot-rolled steel sheet from the steel 2 was less than 6.0, theductility El. and the average r value were insufficient, the Δr waslarge, and the ridging ranks were the D and C ranks.

Nos. 13 and 14 are examples of the present invention in which since thegrain size number of the hot-rolled steel sheet from the steel 2 wasvery decreased, such as 6.5 and 7.1, the average r value wasparticularly improved, the Δr was decreased, and the workability wasimproved.

Nos. 15 and 16 are comparative examples in which the grain size numberof the cold-rolled steel sheet was grown large and coarse, such as 4.5and 5.6, the average r value was large, the ridging was categorized inthe D and C ranks, and the workability was degraded.

Nos. 17, 18 and 19 are examples of the present invention in which sincethe average diameter Dp of the Ti base precipitates, the grain sizenumber of the hot-rolled steel sheet, and the grain size number of thecold-rolled steel sheet were controlled, the average r value was high,and superior workability was obtained.

EXAMPLE 2 Tables 5 and 6

Steel slabs having 10 types of component compositions (steel 5 to steel14) shown in Table 5 which contained various P contents were heated andthen hot-rolled to form hot-rolled steel sheets having a thickness of 4mm. In this example, the precipitation nose temperature T (° C) of theTi base precipitates and the ratio of the precipitated amounts of Ti andP were obtained in the same manner as that in Example 1. Subsequently,the hot-rolled steel sheet was processed by recrystallization annealingat a temperature different from the precipitation nose temperature T asshown in Table 6, and the Ti base precipitates having the averagediameter Dp shown in Table 6 were precipitated. Next, cold rolling wasperformed at a total reduction in thickness of 80% to form a cold-rolledsteel sheet having a thickness of 0.8 mm, and final annealing (annealingof the cold-rolled steel sheet) was then performed at a temperaturedifferent from the precipitation nose temperature T as shown in Table 6.As for the cold-rolled steel sheets thus formed, the grain size, theproperties (YS, TS, El, and r), the ridging, the precipitation ratios ofTi and P, and the refining time were measured in the same manner as thatin Example 1. The results are shown in Table 6.

No. 20 is a comparative example in which the P content was high, such as0.046%, and the inappropriate steel 5 was used having a component systemoutside of the JIS standards. When the P content was too high, althoughthe Ti base precipitates of the hot-rolled steel sheet were grown largeand coarse, the YS was 340 MPa, that is, the high hardness was notchanged.

Nos. 21 to 23 are examples of the present invention in which theappropriate steel 6 to 8 were used. In the examples, when the averagediameter Dp of the Ti base precipitates was set to 0.15 μm to 0.25 μm,although the average diameter Dp indicated very fine grains, a low yieldstrength, a high elongation El. and a high r value were simultaneouslyobtained.

No. 24 is a comparative example in which the inappropriate steel 9 wasused having a decreased P content of 0.008%. When the P content was somuch decreased as described above, although the YS was low, in additionto the increase in anisotropy Δr, the time required for refining becamelonger than that in the past. In addition, when scrap is used in view ofrecycling, there will be a serious limitation.

As is No. 20, No. 25 is a comparative example in which the inappropriatesteel 10 was used having a high P content of 0.042%. Accordingly, the YSwas high, and other mechanical properties were also inferior.

Nos. 26 and 27 are examples of the present invention using theappropriate steel 11 and 12 in which since the average grain diametersDp of the Ti base precipitates were set to 0.22 μm and 0.25 μm, theworkability was improved.

No. 28 is a comparative example using the inappropriate steel 13 inwhich the P content was decreased to 0.005%. In this case, theproperties of the steel were improved; however, the anisotropy Δr wasincreased by grain growth as was expected, and the refining timerequired for reducing the content to 0.005% was very disadvantageouslyincreased. Hence, in view of a recycling process, there is a seriousdisadvantage.

Nos. 29 and 30 are comparative examples using the appropriate steel 7 inwhich the hot-rolled steel sheet was annealed under an annealingcondition outside the range of (a precipitation nose temperature ofTi±50° C.). In No. 29 in which annealing was performed at a temperaturemuch higher than the precipitation nose temperature T, recrystallizationwas advantageously promoted; however, the amounts of C and P in a solidsolution form were increased, and in addition, the size of the Ti baseprecipitates became smaller. As a result, the material was hardened dueto solid solution reinforcement and precipitation reinforcement. On theother hand, in No. 30 in which the annealing temperature was lower thanthe precipitation nose temperature T−70° C., the structure would not berecrystallized at all, or grains would be grown while part of thestructure would remain in a non-recrystallized state. Furthermore, sincethe size of the precipitates is small, superior steel properties couldnot be obtained.

No. 31 is a comparative example in which the Ti base precipitates in thehot-rolled annealed steel sheet were grown large and coarse to have anaverage diameter Dp of 1.11 μm. When the precipitates were grown largeand coarse to have an average diameter Dp of more than 1.0 μm, theductility El. and the average r value were decreased.

No. 32 is a comparative example in which the Ti base precipitates in thehot-rolled annealed steel sheet was grown smaller so as to have anaverage diameter Dp of 0.03 μm. According to the relationship betweenthe average diameter Dp and the yield strength, for example, compared tothe case of No. 22 in which the average diameter Dp in the Ti baseprecipitates was large, the yield strength was large.

No. 33 is an example in which the final annealing temperature was set tothe precipitation nose temperature T+130° C. When the final temperaturewas increased, the Ti base phosphides were redissolved, and hardeningoccurred.

No. 34 is an example of the present invention in which the precipitationnose temperature T−100° C. was satisfied, and in which the ferrite grainsize number of the cold-rolled annealed steel sheet was 6.0 or more.

No. 35 is a comparative example in which since the grain size number ofthe cold-rolled steel sheet was less than 6.0, such as 5.8, the surfaceroughness became apparent, and in which the ridging was categorized inthe rank C.

No. 36 is an example in which grains of the cold-rolled annealed steelsheet were grown large and coarse so that the ferrite grain size numberwas less than 6.0. When the grain diameter of the final-annealed steelsheet was grown large and coarse, the surface roughness became apparentin processing, and the workability was degraded.

No. 37 is an example in which Ti/(C+N) was 5.55 which was much lowerthan a lower limit of 8 defined in the present invention. As the steelwas hardened, and as the ductility El thereof was degraded, thegeneration of ridging apparently occurred.

INDUSTRIAL APPLICABILITY

According to the present invention, in manufacturing a Ti-containingferritic stainless steel sheet having a low yield strength, when a largeamount of P and C remaining in molten steel due to recycling of slag,dust, scrap, and the like is grown in the form of large and coarse Tibase precipitates so as to be harmless materials, a Ti-containingferritic stainless steel sheet having superior ductility and a low YScan be obtained as compared to a conventional material having the samecrystal grain size as that of the steel sheet of the present invention.In addition, since existing machines can be used for manufacturing,recycling and energy saving can be advantageously achieved.

TABLE 1 Chemical Composition(mass) C Si Mn P S Cr Ni N Mo Al Ti Ti/(C +N) 0.003 0.08 0.24 0.024 0.002 15.9 0.11 0.006 0.01 0.01 0.166 18.4

TABLE 2 Hot-rolled annealed steel sheet Average Cold-rolled diameter Dpannealed of Ti base steel sheet Grain size precipitates Vield Samplenumber Dp strength No. (Gs No.) (μm) (MPa) A 5.59 0.28 234 B 6.04 0.28242 C 6.46 0.28 244 D 6.82 0.28 246 E 7.35 0.28 257 F 5.75 0.03 250 G6.18 0.03 260 H 6.71 0.03 265 I 7.00 0.03 274 J 7.36 0.03 280

TABLE 3 Nose temperature Chemical composition (mass %) of Ti base SteelC Si Mn P S Cr Ni N Mo Al Ti Ti/(C + N) precipitates Remarks 1 0.0040.10 0.25 0.046 0.003 16.2 0.11 0.008 0.01 0.02 0.159 13.3 770Comparative example 2 0.004 0.10 0.24 0.038 0.003 16.1 0.12 0.008 0.010.02 0.161 13.4 760 Example 3 0.005 0.11 0.25 0.013 0.003 16.1 0.110.008 0.01 0.02 0.160 12.3 740 Example 4 0.005 0.10 0.25 0.008 0.00316.2 0.11 0.008 0.01 0.02 0.155 11.9 730 Comparative example

TABLE 4 Ratio Average Grain Ratio (%) diam- size (%) of of Grain eternumb- precip- precip- size Dp er itated itated number of Ti of Ti P toof base hot- to total total cold- precip- rolled Ti P rolled AverageSurface Numb- itates steel (mass (mass steel YS TS r Ridging roughnessRefining er Steel μm sheet %) %) sheet MPa MPa El % value Δ r rank μmtime Remarks 1 1 0.12 6.1 60 72 — 280 444 31.8 1.05 0.21 B 0.08 AComparative example 2 2 0.10 6.2 71 75 — 263 429 34.1 1.15 0.13 B 0.10 BExample 3 3 0.11 6.2 69 71 — 250 422 35.3 1.22 0.13 B 0.07 B Example 4 40.12 6.0 55 59 — 243 418 35.6 1.24 0.14 B 0.08 C Comparative example 5 20.03 6.0 40 33 — 281 450 32.5 1.08 0.11 B 0.08 B Comparative example 6 20.07 6.1 61 72 — 265 432 33.6 1.16 0.13 B 0.09 B Example 7 2 0.25 6.1 7255 — 255 430 34.1 1.25 0.15 B 0.11 B Example 8 2 0.61 6.1 75 65 — 253429 34.6 1.21 0.15 B 0.11 B Example 9 2 0.88 6.1 60 73 — 251 429 34.81.16 0.17 B 0.09 B Example 10 2 1.15 6.1 65 68 — 248 425 35.1 1.04 0.15B 0.09 B Comparative example 11 2 0.28 4.5 62 65 — 245 420 31.4 1.040.41 D 0.45 B Comparative example 12 2 0.24 5.5 55 52 — 252 428 34.9 1.20.31 C 0.25 B Comparative example 13 2 0.25 6.5 58 61 — 259 433 34.21.27 0.17 B 0.07 B Example 14 2 0.27 7.1 80 92 — 260 435 33.8 1.31 0.08B 0.05 B Example 15 3 0.11 6.2 61 70 4.5 243 425 30.8 1.69 0.37 D 0.48 BComparative example 16 3 0.11 6.2 55 55 5.6 255 432 34.8 1.9 0.32 C 0.32B Comparative example 17 3 0.11 6.2 62 91 6.2 257 435 34.3 2.03 0.15 B0.08 B Example 18 3 0.11 6.2 55 80 6.8 259 438 33.8 2.01 0.11 B 0.06 BExample 19 3 0.11 6.2 55 71 7.1 262 439 33.1 1.88 0.07 A 0.03 B Example

TABLE 5 Nose Chemical composition (mass %) temperarure of Ti/ Ti baseSteel C Si Mn P S Cr Ni N Mo Al Ti (C + N) precipitates (° C.) Remarks 50.004 0.10 0.25 0.046 0.003 16.2 0.11 0.008 0.01 0.02 0.159 13.3 770Inappropriate steel 6 0.004 0.10 0.24 0.038 0.002 16.1 0.12 0.008 0.010.02 0.161 13.4 760 Appropriate steel 7 0.003 0.08 0.24 0.024 0.002 15.90.11 0.006 0.01 0.01 0.166 18.4 750 Appropriate steel 8 0.005 0.11 0.250.013 0.003 16.1 0.11 0.008 0.01 0.02 0.160 12.3 740 Appropriate steel 90.005 0.10 0.25 0.008 0.003 16.2 0.11 0.008 0.01 0.02 0.155 11.9 730Inappropriate steel 10 0.007 0.25 0.31 0.042 0.002 11.2 0.25 0.009 0.170.03 0.250 15.6 730 Inappropriate steel 11 0.007 0.24 0.30 0.031 0.00211.2 0.24 0.008 0.18 0.03 0.249 16.6 720 Appropriate steel 12 0.006 0.250.31 0.014 0.002 11.1 0.25 0.008 0.18 0.03 0.244 17.4 700 Appropriatesteel 13 0.007 0.25 0.30 0.005 0.002 11.2 0.25 0.007 0.17 0.03 0.25017.9 690 Inappropriate steel 14 0.110 0.08 0.26 0.033 0.002 16.3 0.110.006 0.01 0.01 0.050 5.55 760 Inappropriate steel

TABLE 6 Temperature Temperature difference of difference AverageRatio(%)of Ratio(%)of annealing Grain size of annealing diameterprecipitated precipitated Temperature number temperature Dp of Ti Ti tototal Ti to total of cold-rolled of of hot-rolled base Ti(mass %)(hot-P(mass %)(hot- steel sheet cold-rolled steel sheet precipitates rolledrolled from T steel Number Steel from T ° C. μm steel sheet) steelsheet) ° C. sheet (Gs No.) 20 5 +20 0.30 55 55 +35 6.8 21 6 ±0 0.25 8070 +30 6.7 22 7 ±0 0.15 86 75 +30 7.0 23 8 ±0 0.18 88 95 +30 6.9 24 9 ±00.04 80 80 +30 6.9 25 10 ±0 0.15 88 75 +30 7.2 26 11 ±0 0.22 82 88 +307.1 27 12 ±0 0.25 75 65 +30 6.9 28 13 ±0 0.03 89 80 +30 6.8 29 7 +600.03 30 25 +30 6.7 30 7 −70 0.02 36 40 +30 6.9 31 7 ±0 1.11 70 80 +106.9 32 7 ±0 0.03 80 75 +40 7.0 33 7 ±0 0.22 75 68 +130 6.5 34 7 ±0 0.2268 80 +60 6.8 35 7 ±0 0.22 66 95 +20 5.8 36 7 ±0 0.22 90 88 +30 5.0 3714 ±0 0.13 68 70 +30 6.6 Ratio(%)of Ratio(%)of precipitated precipitatedTi to total Ti to total Ti(mass %) P(mass %) (Cold-rolled (Cold-rolledYS TS Average r Ridging Refining Number steel sheet) steel sheet) MPaMPa El % value Δ r rank time Remarks 20 40 40 340 490 27 1.4 0.21 B AComparative example 21 75 65 273 450 35 1.8 0.19 B B Example 22 83 70265 444 35 1.9 0.22 B B Example 23 65 88 255 435 35 1.7 0.23 B B Example24 70 75 258 439 32 1.6 0.50 B C Comparative example 25 80 68 325 480 311.5 0.19 B A Comparative example 26 75 68 246 426 37 1.9 0.24 B BExample 27 68 59 240 420 40 2.1 0.24 B B Example 28 86 75 243 422 35 1.90.55 B C Comparative example 29 50 43 280 450 34.5 1.6 0.22 B BComparative example 30 40 45 320 500 34.3 1.2 0.13 C B Comparativeexample 31 60 90 248 418 29 1.18 0.55 B B Comparative example 32 55 75281 455 34 1.55 0.21 B B Comparative example 33 70 65 293 440 35 1.660.29 B B Comparative example 34 70 80 297 441 34.3 1.55 0.26 B B Example35 65 92 241 420 38 1.9 0.15 C B Comparative example 36 70 80 237 412 402.0 0.17 D B Comparative example 37 60 55 285 510 25 1.1 0.39 D BComparative example

1. A Ti-containing ferritic stainless steel sheet comprising on a masspercent basis: 0.01% or less of C; 0.5% or less of Si; 0.3% or less ofMn; 0.01% to 0.04% of P; 0.01% or less of S; 8% to 30% of Cr; 1.0% orless of Al; 0.05% to 0.5% of Ti; 0.04% or less of N, 8≦Ti/(C+N)≦30 beingsatisfied; and being free of Nb, with the balance being substantially Feand incidental impurities, wherein at least 50% of the total P contentin the steel sheet is precipitated in the form of the Ti baseprecipitates, a grain size number of ferrite grain is 6.0 or more, andan average diameter Dp of precipitations, each being [(a long axislength of a Ti base precipitate+a short axis length thereof)/2], of theTi base precipitates in the steel sheet is in the range of from 0.05 μmto 1.0 μm.
 2. The Ti-containing ferritic stainless steel sheet accordingto claim 1, wherein at least 50% of the total Ti content in the steelsheet is precipitated in the form of the Ti base precipitates.
 3. TheTi-containing ferritic stainless steel sheet according to one of claims1 to 2, wherein the steel sheet is a hot-rolled steel sheet.
 4. TheTi-containing ferritic stainless steel sheet according to one of claims1 to 2, wherein the steel sheet is a cold-rolled steel sheet.
 5. Amethod for manufacturing a Ti-containing ferritic stainless steel sheetcomprising the steps of: hot-rolling steel which comprises on a masspercent basis: 0.01% or less of C; 0.5% or less of Si; 0.3% or less ofMn; 0.01% to 0.04% of P; 0.01% or less of S; 8% to 30% of Cr; 1.0% orless of Al; 0.05% to 0.5% of Ti; 0.04% or less of N, 8≦Ti/(C+N)≦30 beingsatisfied; and being free of Nb, with the balance being substantially Feand incidental impurities, for forming a hot-rolled steel sheet, andperforming recrystallization annealing of the hot-rolled steel sheet ata temperature of (a precipitation nose temperature of Ti baseprecipitates±50° C.) so that an average diameter Dp of precipitationdiameters, each being [(a long axis length of a Ti base precipitate+ashort axis length thereof)/2], of the Ti base precipitates in the steelsheet is in the range of from 0.05 μm to 1.0 μm and so that a grain sizenumber of ferrite grain is 6.0 or more and such that at least 50% of thetotal P content in the steel sheet is precipitated in the form of the Tibase precipitates.
 6. The Ti-containing ferritic stainless steel sheetaccording to claim 5, wherein at least 50% of the total Ti content inthe steel sheet is precipitated in the form of the Ti base precipitates.7. The method for manufacturing a Ti-containing ferritic stainless steelsheet, according to claim 5, further comprising the steps of:cold-rolling the hot-rolled annealed steel sheet; and subsequentlyperforming final annealing of the cold-rolled steel sheet at atemperature less than (the precipitation nose temperature of Ti baseprecipitates+100° C.) so that the average diameter Dp of precipitationdiameters, each being [(a long axis length of a Ti base precipitate+ashort axis length thereof)/2], of the Ti base precipitates is in therange of from 0.05 μm to 1.0 μm and so that the grain size number offerrite grain is 6.0 or more.
 8. The method for manufacturing aTi-containing ferritic stainless steel sheet, according to claim 7,wherein the final annealing is performed at a temperature less than (theprecipitation nose temperature of Ti base precipitates+50° C.).
 9. Themethod for manufacturing a Ti-containing ferritic stainless steel sheet,according to claim 7 or 8, wherein at least 50% of the total Ti contentin the steel sheet is precipitated in the form of the Ti baseprecipitates.
 10. The steel according to claim 1, further comprising atleast one of 0.3% or less of Ni, 0.3% or less of Cu, 0.3% or less of Co,0.5% or less of Zr, 0.1% or less of Ca, 0.3% or less of Ta, 0.3% or lessof W, 0.3% or less of V, 0.3% or less of Sn, 2.0% or less of Mo and0.003% or less of Mg.
 11. The method according to claim 5, wherein thesheet further comprises at least one of 0.3% or less of Ni, 0.3% or lessof Cu, 0.3% or less of Co, 0.5% or less of Zr, 0.1% or less of Ca, 0.3%or less of Ta, 0.3% or less of W, 0.3% or less of V, 0.3% or less of Sn,2.0% or less of Mo and 0.003% or less of Mg.